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Theoretical Limit Calculated for 'Ultimate PC'

If your laptop computer can't keep up with your most urgent computational needs, cheer up. A physicist has calculated how to make PCs almost unimaginably faster--although to reach this theoretical speed limit, they'd have to be as dense as black holes.

Seth Lloyd, a physicist at the Massachusetts Institute of Technology, calculated the ultimate physical limits on the speed of a computer using the laws of thermodynamics, information theory, relativity, and quantum mechanics. For starters, he used information theory to show that a 1-kilogram, 1-liter laptop could store and process at most 1031 bits of information. (A nice-sized hard drive holds about 1011 bits.)

Then Lloyd figured out how quickly it could manipulate those bits. To do this, he invoked Heisenberg's Uncertainty Principle, which implies that the more energy a system has available, the faster it can flip bits. Lloyd's ultimate laptop would convert all of its 1-kilogram mass into energy via Einstein's famous equation E = mc2, thus turning itself into a billion-degree blob of plasma. "This would present a packaging problem," Lloyd admits with a laugh. If this energetic lump could somehow be turned into a computer, it could perform 1051 operations per second, he reports in the 31 August issue of Nature. By comparison, today's planned peak performer will be able to do 1013 operations per second.

But processing speed is only half of the story. If you really want to speed up your computer, Lloyd says, you must also slash the time it takes to communicate with itself--that is, to transfer information back and forth. The trick, he says, is to squeeze the computer down to the most compact possible size. Lloyd shows that a computer made of the most compressed matter in the universe--a black hole--would calculate as fast as a plasma computer. It would also communicate in precisely the same time that previous calculations showed is necessary to flip a bit--the hallmark of the ideal computer. Coincidence? Maybe not. "Something really deep might be going on," Lloyd remarks.

At present, scientists have no idea how to turn a laptop into a black hole (Windows 98 jokes aside). But Raymond Laflamme, a physicist at the Los Alamos National Laboratory in New Mexico, says that just thinking about such extreme scenarios might illuminate mysteries such as black holes.